It has long been known to log open boreholes to measure acoustic wave energy traveling through rock formations located in the borehole region. Logging devices have been used for this purpose which normally comprise a sound source (transmitter) and one or more receivers disposed at preselected distances from the sound source. By measuring the travel of such acoustic waves between the transmitter and one or more receivers, it is normally possible to determine the nature of surrounding rock formations. In logging loosely consolidated formations, however, it is often difficult to distinguish between compressional, shear, tube and secondary waves which may comprise portions of a wave train arriving at a given receiver. The use of remotely spaced, multiple receivers is thus intended to aid in distinguishing between arriving wave fronts and noise in the system. Multiple receivers permit the recognition of similar wave patterns and wave fronts which are received at each successive receiver. Since travel time differentials increase with increasing distance from the transmitter source, wave fronts and patterns which are closely spaced at proximate receiver locations will separate by the time of their receipt at remote receiver locations.
Various signal timing and wave front analysis methods have also been suggested for distinguishing between wave fronts received at a given receiver. Most of these methods involve timing circuits which anticipate the receipt of, and facilitate the collection of, such wave front information. For descriptions of various logging techniques for collecting and analyzing compressional wave, shear wave, tube wave, and secondary wave data, please refer to U.S. Pat. No. 3,333,238 (Caldwell), U.S. Pat. No. 3,362,011 (Zemanek, Jr.), Reissue U.S. Pat. No. 24,446 (Summers), and U.S. Pat. No. 4,383,308 (Caldwell).
In the design of logging tools, various types of transmitters, such as piezoelectric or magnetostrictive transmitters, have been suggested for creating acoustic logging signals. For conventional logging operations, most such transmitters have been centrally located in the borehole and have been adapted to generate sound which is radiated in a multidirectional (360.degree.) pattern from the transmitter to adjacent wellbore surfaces.
Recently, attention has been directed to developing transmitters which are particularly suited to a single point force application of acoustic energy to the borehole wall. The theory behind point force transmitters is that they produce an asymmetrical acoustic energy radiation pattern as contrasted with the multidirectional radiation pattern. One such point force transmitter is the bender-type disclosed in Canadian Pat. No. 1,152,201 (Angona and Zemanek).